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A spokeswoman for Germany's federal state prosecutor in Karlsruhe, Frauke Koehler, told the BBC: "Two suspects from the Islamist spectrum have become the focus of our investigation. Both of their apartments were searched, and one of the two has been detained."

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The letter, published online, said the threat against high-profile public figures will last until German planes are withdrawn from the war zone, and the US air base at Ramstein, near the French border, is closed.

Football fans are not mentioned as targets, and there is no signature.

The club has previously sought to clamp down on far-right groups in its fan base.

Social workers were brought in to deal with neo-Nazi activists amid unrest over immigration levels in the Ruhr Valley, in the west of the country.